Polypropylene nucleation density

Observations of polypropylene crystallization by polarized light microscopy helped to understand phenomena on interphase of polymer-filler. When analyzing the influence of the filler, many researchers noted transciystallization of the polypropylene as a result of high enough nucleation density rai the filler s surface and also in the presence polypropylene fibers. The addition of both mineral substances and natural composites containing lignocellulosic material can induce formation of TCL. 

The effect of flax fiber modification and the effect on the presence of TCL were also studied by Arbelaiz et al. [17]. In his studies, fibers were subjected to maleic anhydride, vinyltrimetoxy silane, and aUcali with addition of MAPP-grafted polypropylene. All applied modifications produced improvement in the thermal stability of lignocellulosic material. Analysis of the crystalline structure showed the increase of crystallinity as a result of fiUer addition. Nucleation density around unmodified fibers was higher than around fibers modified with maleic anhydride. Surfaces of unmodified fibers had the surface active for initialization of transcrystaUization, which, in the case of modified fibers, did not occur. Son et al. [32] explained this phenomenon as... 

The thermal and nncleation behavior of a-PVDF depends on its microstmcture. Samples with higher head-to-head inversions display a significarrt memory effect." Their initial spherulitic morphology is maintained in snccessive recrystaUizatiorrs." The nucleation density can be increased by addition of nncleating agents PTFE or flavanthrone." The nuclei densities in PVDF is nearly two orders of magnitude lower than that for isotactic polypropylene." ... 

Table 1 Sphemlite nucleation density of isotactic polypropylene of A4 =51 200, heptane extracted, melt annealed before crystallization at 180°C for 15 min. (von Falkai, B. and Stuart, H.A. (1959) Kolloid Zeitschrift, 162,138-140)...

The measured T]/2 values of many polypropylene samples of high isotacticity are shown in Fig. 14 as a function of T. It is seen that tj/2 increases very rapidly with Jc whereas at any given T, no statistically significant correlation with molecular mass is found. The independence of Ti/2 with molecular mass agrees with the fact that nucleation density and spherulite radial growth rate were found to have no identifiable dependence on molecular mass. 

The nonlinearity is also seen in the experimental results presented in Fig. 5.57. Nucleation densities c uc in i-polypropylene were again determined after shearing pulses of different lengths producing different amounts of shear Aej x, now also at various temperatures. The dependence of c uc on the mechanical work can be represented by a power law with an exponent... 

Term N J, Fairclough P A, Towns-Andrews E, Komanshek B U, Young R J and Ryan A J 1998 Density fluctuations the nucleation event in isotactic polypropylene crystallization Polymer 29 2381- 5... 

Polypropylene molecules repeatedly fold upon themselves to form lamellae, the sizes of which ate a function of the crystallisa tion conditions. Higher degrees of order are obtained upon formation of crystalline aggregates, or spheruHtes. The presence of a central crystallisation nucleus from which the lamellae radiate is clearly evident in these stmctures. Observations using cross-polarized light illustrates the characteristic Maltese cross model (Fig. 2b). The optical and mechanical properties ate a function of the size and number of spheruHtes and can be modified by nucleating agents. Crystallinity can also be inferred from thermal analysis (28) and density measurements (29). 

Polypropylene has a crystal growth rate some one hundred times lower than that of high density polyethylene for example. It therefore responds well to heterogeneous nucleation by well dispersed additives present in the crystallizing melt. 

Wood fiour is often added to thermoplastics as a low cost filler to alter mechanical performance, especially the stiffness of low melt temperature, commodity thermoplastics such as polypropylene and polyethylene without increasing density excessively. Wood is much stiffer than the commodity thermoplastics usually used as matrices. Additionally, vood and pulp fibers can nucleate crystal growth in polyolefins resulting in a transcrystalline layer that can infiuence mechanical behavior [33, 34[. 

Hyperfom HPN 210 M is a new nucleating agent for injection molded high-density polyethylene apphcations. It improves mechanical performance to the levels approaching those of polypropylene impact copolymers. It increases stiffness by up to 50%, heat distortion temperature by 10-40°C, and productivity. It has ability to create lamellar crystal orientation of PE in the polymer flow direction reducing shrinkage. ... 

A hot melt adhesive composition made from polypropylene copolymer or polypropylene impact copolymer, a polyolefin elastomer, a low density polyethylene, a tackilying resin, a plasticizer, and a nucleating agent The 0.2 to 1 wt% of nucleating agent is... 

Bartczak Z. Galeski A. PraceUa M. Spherulite nucleation in blends of isotactic polypropylene with high density polyethylene. Polymer. 1986,27,537-543. 

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